AVS2014 Session MN+NS-TuA: Multi-Scale Phenomena and Bio-Inspired MEMS/NEMS

Three-dimensional integrated circuit (3D-IC) technology has been developed using copper (Cu) filled through silicon vias (TSVs). The vertical interconnects pass through a set of stacked die and enable many applications that benefit from increased bandwidth, reduced signal delay, and improved power management. However, the reliability of Cu interconnects is a serious concern since the performance is affected by electromigration and stress associated with mismatch in thermal expansion coefficients. Carbon nanotubes (CNTs) are nanoscale materials which possess a high Young's modulus, a low coefficient of thermal expansion, and high thermal conductivity. Compared to Cu, CNTs exhibit low resistivity due to the existence of ballistic conduction and they are capable of carrying a higher current density. In this work, we fabricated TSVs using a novel materials system consisting of a composite of Cu and CNTs as a possible solution to the problems encountered in Cu-based interconnects. First, blind TSVs were fabricated using a Bosch process. After etching, an insulating layer, a metallic seed layer, and a catalyst layer were deposited previous to CNT growth. Vertically aligned CNTs were grown by chemical vapor deposition method. Finally, Cu was deposited by periodic reverse pulse electroplating inside the vias to form a Cu/CNT composite. Polishing completed the fabrication and allowed measurement of electrical performance for TSV interconnects. The experimental results were compared for interconnects filled with Cu and those filled with the Cu/CNT composite. The results are encouraging for the Cu/CNT composite having potential application as a TSV interconnect material.

Meso scale microelectromechanical structures found on the upper scale of microelectromechanical systems (MEMS) can potentially replace conventional mechanical devices produced by common for the macro-engineering approaches such as machining and assembly of individual parts. When realized as a compliant mechanism containing a single flexible member rather than multiple parts attached by joints they provide smooth frictionless motion without backlash and exhibit improved reliability and robustness. Batch fabrication using micromachining processes established in MEMS allows improved yield and significantly lower cost. However, actuation of these devices remains challenging. Electrostatic actuation, which is the most widely used in smaller MEMS devices, is viewed to be less suitable for the actuation at the meso scale due to unfavorable scaling laws, namely quadratic reduction of the actuating force with the distance between the electrodes. For this reason, most of the meso scale micro devices are actuated by thermal transducers, distinguished by slow response and high power consumption or by piezoelectric or magnetic motors, which cannot be integrated within the device and require post-fabrication assembly.

In this work we report on the design, fabrication and characterization of an electrostatically actuated meso scale microelectromechanical motion transformer and amplifier. The actuator corporate a transducer with multiple parallel plate electrodes and is realized as a compliant mechanism relying on flexible pseudo hinges. The 5000 µm × 4000 µm device converts linear motion of the transducer into mechanically amplified angular motion of a rotating lever. By combining a highly efficient small-gap parallel plate electrode and a motion amplification the device is designed to provide a lever tip displacement of 60µm, an initial blocking force of 0.001N at zero displacements and a blocking force of 0.012N in the maximal displacement configuration when the parallel plate actuator is in its closed position. The devices were fabricated using DRIE from a SOI wafer with (111) front surface orientation and a 150µm thick device layer. The devices were operated in ambient air conditions and the functionality of the device was demonstrated experimentally. The voltage-displacement dependence and resonant curves were built using image processing procedure implemented in Matlab. Excellent agreement between the results provided by the Finite Elements models and the experimental data was observed. The results of the work demonstrate an ability to achieve both large displacements and high blocking forces in an electrostatically actuated meso scale compliant mechanism.

We present a multi-experiment PDMS based biofilm analysis platform using a valve-actuated microfluidic system, designed to reduce growth variance of in-vitro biofilms to less than 10%. This was achieved by integrating hydraulic push-down valve actuators to section a uniform biofilm grown in a channel by maintaining a single source of bacterial suspension [1, 2]. In this work, we establish a simplified process flow for the fabrication of a multi-depth device mold and demonstrate the high throughput capability of the microfluidic biofilm reactor.

Bacterial biofilms are the primary cause of infections in medical implants and catheters. The widespread use of high doses of antibiotics to treat biofilm infections is leading to the emergence of antibiotic resistant strains, necessitating the development of alternative methods of treatment [3]. However, the experimental evaluation of new treatment techniques is strongly hindered by the stochastic nature of biofilm growth [1]. Therefore, it is required to develop a microsystem that can not only facilitate multi-experiment studies for new treatment evaluation but also enable the growth of uniform biofilms that can be used as reliable controls.

Figure 1 shows a single uniform biofilm grown in the horizontal center channel of the device is sectioned into multiple sections, by hydraulically actuating the push-down valves thereby enabling multi-experiment studies on the same biofilm. Figure 2 shows the schematic of the operation of a “push-down” valve [4], the schematic of the CAD layout of the two-level microfluidic device and its two modes of operation. The two-step photolithography of the molds (Figure 3) using negative photoresists, SU8-2015 and KMPR1050, allows for the patterning of the multi-depth microfluidic mold without the need for additional passivation of the first resist layer, thereby simplifying fabrication. The mold can be reused to produce multiple devices; a photograph of the valve region of a used multi-depth mold is shown in Figure 4. Photographs of the device operating in different modes are shown in Figure 5.

The unique capability of this valved microfluidic biofilm reactor to section uniform biofilms can facilitate high-throughput biofilm studies, including new drug discovery. The push-down valve configuration allows for easy integration of electrodes for the study of alternative treatment methods like electric fields. Furthermore, the integration of the biofilm reactor with a real-time measurement system will enable high-throughput continuous analyses on uniform biofilms while ensuring tight and reliable controls.

We experimentally demonstrate, for the first time to our knowledge, the operation of silicon carbide (SiC) microdisk resonators in fluidic and viscous environments (particularly in water) with robust multiple flexural-mode resonances in the high and very high frequency (HF/VHF) radio band. We observe ~8 resonance modes in a 20µm-in-diameter SiC microdisk resonator with resonance frequencies up to ~120 MHz and quality Q factors as high as ~40 in water.

SiC is a highly attractive material for microelectromechanical systems (MEMS) due to its superior mechanical (e.g. high elastic modulus, E_Y~450 GPa), optical (wide bandgap, >2.3eV) and thermal properties (thermal conductivity of 320-490 W/[m×K]) ^[1].These advantages make SiC especially suitable for sensing applications in liquid for its transparency from visible to mid-infrared light and high optical power handling ability, facilitating efficient laser actuation and detection. Meanwhile, two dimensional (2D) microdisk structure exhibits multiple flexural-mode resonance characteristics, which can enhance sensing performances in liquid with the additional degrees of freedom and larger sensing area. Further, the unique biocompatibility of SiC allows potential in-liquid biosensing applications be developed.

In this study, we demonstrate the operation of high frequency SiC microdisk resonators in liquid. The SiC microdisk resonators are completely immersed in water, and are optically driven by an amplitude-modulated 405nm laser. The multimode resonances are detected with optical interferometry using a 603nm He-Ne laser. We observe ~8 resonance modes up to ~120 MHz with Qs as high as ~40 in water. To our best knowledge, both the number of resonance modes and Qs measured are the highest among flexural-mode resonators operating in water reported to date^{[2],[3],[4],[5]}. Such high frequency SiC microdisk resonators with robust multimode resonances and high Qs in water may provide an appealing platform for particle and biological sensing applications in liquid.